The present invention relates to an apparatus for forming multicolor electrophotographic images by employing a wet-type developing process.
Conventional apparatus for forming multicolor electrophotographic images through the wet-type developing process have as many developing devices as the number of the colors of developers, the developing devices employing dish-shaped development electrodes. The developing device having such a dish-shaped development electrode is disclosed in Japanese Patent Publication No. 56-16424 (Japanese Laid-Open Patent Publication No. 50-17844), Japanese Patent Publication No. 54-43898 (Japanese Laid-Open Patent Publication No. 50-17843), Japanese Laid-Open Patent Publication No. 52-82241, Japanese Patent Publication No. 58-32376 (Japanese Laid-Open Patent Publication No. 51-78335), Japanese Laid-Open Patent Publication No. 53-26137, and Japanese Laid-Open Patent Publication No. 53-53334, for example. Where the dish-shaped electrode is used, difficulty has been experienced in maintaining a desired gap between itself and a latent image carrier or photosensitive drum, and toner tends to get attached to the photosensitive drum when the electrode is cleaned. Therefore, the cleaning device operates under an increased burden, and the toner is cosumed and degraded at a high rate. Another problem is that the image forming apparatus is large in size since it has as many developing devices as there are developers required. The developing system employing the dish-shaped electrode does not have a means such as a scraper for mechanically removing the developer which has been returned to the dish-shaped electrode under a developing bias to clear the image background. Therefore, the developer is transferred back to the photosensitive drum by applying a reverse bias, and the developer is then removed from the photosensitive drum by the cleaning device. The amount of developer to be removed by the cleaning device is however quite large. If the developer were returned to a developer tank, then the amount of the toner which has been degraded by the reverse bias would be increased, and there would eventually arise the problem of color mixing. To avoid such drawbacks, the collected developer should not be recycled, thus resulting in increased toner consumption.
For developing fully colored images, it is necessary to suppress the edge effect so as to attain desired hues by superposing the colors. Altough it would be preferable to locate the development electrode closely to the photosensitive drum to meet the above requirement, positioning the dish-shaped development element closely to the photosensitive drum would permit the liquid developer to leak due to capillary action, thus causing color mixing. Therefore, the dish-shaped development electrode could not be positioned closely to the photosensitive drum.